Some new designs of mobile communication devices-such as smart phones, tablet computers, and laptop computers-contain one or more Subscriber Identity Module (“SIM”) modules (e.g., cards) that provide users with access to multiple separate mobile telephony networks. Examples of mobile telephony networks include GSM, LTE, TD-SCDMA, CDMA2000, and WCDMA. A mobile communication device that includes one or more SIMs and connects to two or more separate mobile telephony networks using one or more shared radio frequency (“RF”) resources/radios is termed a multi-SIM communication device. One example is a dual-SIM dual standby (“DSDS”) communication device, which includes two SIM cards/subscriptions that are each associated with a separate radio access technology (“RAT”), and the separate RATs share one RF chain to communicate with two separate mobile telephony networks on behalf of their respective subscriptions. When one RAT is using the RF resource, the other RAT is in stand-by mode and is not able to communicate using the RF resource.
One consequence of having a plurality of RATs that maintain network connections simultaneously is that the RATs may sometimes interfere with each other's communications. For example, two RATs on a DSDS communication device utilize a shared RF resource to communicate with their respective mobile telephony networks, and only one RAT may use the RF resource to communicate with the RAT's mobile network at a time. Even when a RAT is in an “idle-standby” mode, meaning that the RAT is not actively communicating with the network, the RAT may still need to periodically receive access to the shared RF resource in order to perform various network operations. For example, an idle RAT may need the shared RF resource at regular intervals to perform idle-mode operations to receive network paging messages in order to remain connected to the network on behalf of the RAT's subscription.
In conventional multi-SIM communication devices, the RAT actively using an RF resource that is shared with an idle RAT may occasionally be forced to interrupt the active RAT's RF operations so that the idle RAT may use the shared RF resource to perform the idle RAT's idle-standby mode operations (e.g., paging monitoring, cell reselection, system information monitoring, etc.). This process of switching access of the shared RF resource from the active RAT to the idle RAT is sometimes referred to as a “tune-away,” as the RF resource tunes away from the active RAT's frequency band or channel and tune to the idle RAT's frequency bands or channels. After the idle RAT has finished network communications, access to the RF resource may switch from the idle RAT to the active RAT via a “tune-back” operation.
A network base station may utilize timing advance values to communicate with mobile communication devices that are camped on the base station. Timing advance values are used by a mobile communication device to adjust for signal propagation delays that occur due to a number of factors, including differences in distance between the mobile communication devices and the base station. For example, a base station may communicate with two mobile communication devices, one located next to the base station and other located five kilometers away from the base station. Uplink communications sent by the mobile communication device next to the base station are received by the base station almost instantaneously. However, uplink communications sent by the mobile communication device five kilometers away arrive at the base station after a certain delay period because of the distance traveled by the uplink signal.
Network base stations usually assign specific time slots in which to receive communications from each mobile communication device camped on the base station. If all the mobile communication devices are the same distance from the base station, then the base station receives communications from each mobile communication device without conflict. However, when the mobile communication devices are varying distances from the base station, the base station may receive one mobile communication device's uplink communication before the base station is finished receiving another device's uplink communication. The base station utilizes timing advances to avoid reception conflicts. The base station calculates a timing advance adjustment value for each mobile communication device and sends the adjustment values to each mobile communication device. The timing advance adjustment value is based on a change in the transmit time (e.g., due to a change in distance) between the mobile communication device and the base station since the last time the timing advance was determined. Each mobile communication device offsets the timing of communications with the base station by the received timing advance adjustment value so that the base station receives communications from all mobile communication devices camped on the base station at the appropriate time. As a mobile communication device moves toward or away from the base station, the base station may periodically send timing advance adjustment values that further adjust the prior timing advance values on the mobile communication device.
In some instances, the base station may calculate an erroneous timing advance adjustment value. This may occur, for example, after a tune-away by the mobile communication device to another network. In such instances, the base station may send the erroneous timing advance adjustment value to the mobile communication device. If the mobile communication device adjusts the prior timing advance value using the erroneous timing advance adjustment value, the subsequent uplink communications from the mobile communication device may conflict with communications from other devices at the base station. This may result in loss of the uplink connection between the mobile communication device and the base station. In this situation, it may take a long time for the mobile communication device and the base station to reestablish the uplink connection.